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I have a two-wire thermostat and an old forced air gas furnace in my house. When you open up the cabinet, inside you find a Dayton 2E228 Gas Valve and it is wired to an external 24VAC Transformer apparently "in series" with the thermostat. Here's a schematic of what I traced out:

enter image description here

I got an Ecobee3, and discovered to my disappointment it requires a third wire in the thermostat cable for a 24VAC transformer common (C) to power it. Following the trail, I went off and bought a Fast-Stat Commonmaker to try and solve the problem. You look at the wiring diagram that they propose in the manual for a system ostensibly like mine with a gas valve, and it doesn't really resemble what I've got. The valve in their example is wired in parallel with the transformer:

enter image description here

I tried to rewire the gas valve + transformer + Commonmaker according to that diagram assuming that the TH terminals correspond to the THERM side of my gas valve, and the TR terminals correspond to the TRANS side of my gas valve, but my gas valve must be different somehow from what the Commonmaker instructions are describing. I did manage to get the Ecobee3 to power up after doing that, but it was not turning my furnace on.

I've put everything back the way it was with my old thermostat so that I don't freeze overnight and it seems to be working. Could anyone offer any advice on how to correctly wire the Commonmaker into my primitive / old HVAC system such that my Ecobee3 thermostat can both power up and control the temperature in my home?

Here's a picture of the gas valve: enter image description here

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  • Can you post photos of the wiring at your gas valve please? Jan 30, 2023 at 2:13
  • @ThreePhaseEel Update: posted a picture of the gas valve
    – vicatcu
    Jan 30, 2023 at 2:14

2 Answers 2

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I got it figured out. I ended up calling a heating technician to come out and help, but I actually ended up doing the wiring myself essentially while he watched. The key observation was that there is actually a different wiring diagram in the common maker manual that more closely (exactly actually) resembled my existing wiring, labelled "Externally Wired Equipment" here:

enter image description here

Caveat the swapping of R and W in the originally wiring as pointed out in the other answer. I re-wired it exactly like that and it just worked.

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Transformer R needs to go to thermostat R

Your first problem is that you have your existing thermostat wired "backwards" with the transformer hot (R) wire connected to the thermostat's W terminal and the thermostat R terminals connected to the gas valve 1, or TH in the Ecobee diagram, terminal. Mechanical thermostats don't care about this, but your Ecobee won't be happy until you switch the two around, even if you manage to get a C wire up there.

Once that is done, the idea behind most gas valves is that they provide a spare terminal or pair of linked terminals that let R "pass through" the valve. This is the 2-3 pair on your valve, or TR-TH in the common nomenclature, and can be used or not used at your discretion -- if you don't use it, simply splice the red wire on the Fast-Stat receiver into the existing R wire with a suitably sized wirenut. Once you have R hooked up to the receiver, the wiring from there follows the diagram in the manual that you posted.

This leaves terminal 4, which is the TR terminal on your gas valve and thus a suitable terminal for landing a C wire onto.

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  • Interesting... I'm not sure I completely follow to be honest. When I completely re-wired the transformer, thermostat cable, and gas valve to match the Commonmaker schematic I thought I did it right. The thing I guess I'm most unsure of is how the numbered terminals on my valve relate to the (unnumbered) terminals in the Commonmaker diagram.
    – vicatcu
    Jan 30, 2023 at 2:30
  • Also I'm assuming the transformer wires are not polarized / can be wired either way around, right?
    – vicatcu
    Jan 30, 2023 at 2:35
  • @vicatcu -- not inherently, no. (some furnaces have an external ground jumper from one side of the transformer to the furnace metalwork) Jan 30, 2023 at 3:32
  • So if I cut power to the transformer, I'm thinking I can check for continuity between either transformer contact and the nearby metal electrical box housing the switch. If neither of them rings out, then neither of the transformer terminals is tied to ground. Likewise, with power cut, I can check for continuity between contacts 2 and 3 of the gas valve and they should ring out. I'm not certain whether any other pairs of contacts in the gas valve should ring out or not, but I'm guessing not. Are these sensible checks to do?
    – vicatcu
    Jan 30, 2023 at 15:36
  • @vicatcu -- yeah, they're quite sensible checks. 1-4 should have some resistance between them, not sure if it'll be low enough to ring your continuity tester though, as that's the gas valve's operating coil Jan 31, 2023 at 5:08

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